In the prior art a device has been proposed that is mounted to a vehicle, that detects other vehicles from an image that has been photographed with a camera, and that outputs a warning when there is a possibility of a collision with the subject vehicle. In relation to this type of device, a technique is per se known that detects regions of reflection from the road surface due to headlights in the image, and that, by performing processing while eliminating these regions, detects the fronts of vehicles and the rears of vehicles accurately, even when such reflections from the road surface due to headlights have spread out and become merged together (refer to Patent Document #1).